The present invention relates to a method for removing an acidic deposit attached to an apparatus for a combustion process, such as a boiler, etc.
When a fuel containing a sulfur component is burned by a combustion process such as a boiler, an acidic deposit containing a sulfur compound will usually form at a portion of a piping or an apparatus disposed between a combustion furnace and a chimney, where a high temperature exhaust gas (hereinafter referred to as the exhaust gas) formed during the combustion, will contact. Especially in a boiler, an apparatus (hereinafter referred to as an heat exchanger) for heat exchange between the exhaust gas and a low temperature air for combustion, is installed in order to improve the combustion efficiency and to prevent corrosion, whereby such an acidic deposit is likely to form.
Usually, in the operation of a boiler, the temperature of the exhaust gas is higher than the dew point of sulfuric acid, and a sulfur compound such as SO3 (hereinafter referred to as a SO3 component) contained in the exhaust gas will not condense as sulfuric acid in a piping or an apparatus (hereinafter referred to simply as in an apparatus) disposed between the combustion furnace and the chimney. However, once the operation of the boiler is stopped, the interior of the apparatus becomes lower than the dew point of sulfuric acid, and the SO3 component in the exhaust gas will condense and attach as sulfuric acid in the apparatus. And, this sulfuric acid will react with at least one component selected from the group consisting of ammonium, sodium, potassium, magnesium, calcium and vanadium, contained in the fuel oil or added during combustion, to form an acidic solid salt such as a hydrogensulfate represented, for example, by ammonium hydrogensulfate, and this hydrogensulfate will be mixed with a dust and will attach in the apparatus. This substance attached in the apparatus is referred to as an acidic deposit. The acidic substance further includes other acidic substances such as hydrochloric acid, nitric acid and sulfuric acid which may be formed depending upon the fuel, the combustion method and the combustion conditions, and further, the acidic deposit may contain iron rust, dust and soot which are insoluble in water.
If such an acidic deposit remains in the apparatus for a combustion process, such as a boiler, there will be problem that as the combustion operation continues, it will hinder the flow of gas, and it will bring about corrosion of a metal such as iron in the apparatus. Accordingly, it is necessary to periodically remove such an acidic deposit.
Heretofore, in the case of a heat exchanger in a combustion process such as a boiler, it has been common to carry out removal of an acidic deposit by a method of washing with water, heat storage elements (hereinafter referred to simply as elements) constituting the regenerative heat exchanger after or without dismounting them.
However, if this acidic deposit is washed with water, the sulfuric acid component in the acidic deposit will dissolve in water to form an acidic aqueous solution. Especially in the case of a heat exchanger, the shapes of the elements are complex, and there will be a problem such that even if a large amount of water is used for washing, dilute sulfuric acid is likely to remain at corners of the elements. Thus, due to sulfuric acid which will form at the time of washing the acidic deposit with water, corrosion is likely to result at the heat exchanger elements or at metallic portions to be used for instruments disposed in a flue, such as a valve or a dust removing equipment, thus leading to a serious problem with respect to the useful life of the installation or stable continuous operation.
Under these circumstances, it is an object of the present invention to provide a method whereby the acidic deposit in a combustion apparatus such as a boiler can be removed easily, safely and in a short time, and the amount of waste water can be reduced.
The present invention provides a method for removing an acidic deposit containing a sulfur compound, which comprises contacting the acidic deposit with an aqueous solution of an alkali metal carbonate and/or an alkali metal hydrogencarbonate to remove it.